>I'm just gonna fill the world with buried treasure
Then just put it on an actual Shovel. So I have to carry one around, and can only use it to slowly dig a bit of a Hole, not full on Terrain modification. That way you can mask it better with some hole-model, and avoid it becoming a cheese-able mechanic. Similarly to how in other Games the chest rises out of the floor instead of the floor sinking down, just accurately reversed. (so reversed Reversed kind of, hope its clear what I mean. You'll still need the system for that, but you won't be able to actually use/abuse it as a Player)
>Model/Graphic
You don't have connected Shoulders, there shouldn't be any Problems arising with shoulder size. If you mean that they don't appear in the Players Camera in first person, use a different Model for that then.
Depending on how long it takes you to get around to the Graphics, there might also be quite a few new ones around you could just download and use, I'm pretty much always trying to make Stuff around that Area of Style.
>Lag
It didn't happen with Trees or anything, and I doubt 30 Skeletal Meshes caused that, I'm pretty sure that it'd be your AI. Do you do Visionchecks etc. every frame or on slower intervalls maybe?